Silver-studded blue and northern blue

From a delight to a bother

Photo by Jan W. Alhfors

Silver-studded blue and northern blue (Plebejus argus ja Plebejus idas) resemble each other so closely that it is no shame to mix them up. The males of these butterflies usually have bright blue top surface of their wings, whereas females have brown wings, although northern blue females tend to often have some blue colour mixed in with the brown.

These species also resemble each other in other ways: they are often seen in similar, sunny areas of coniferous forests, such as along forest roads and paths, peatlands and dry meadows. Damp ground on a dirt road left behind by a puddle that has evaporated in the heat of summer will attract these day-time butterflies like a magnet. They can make do with flowers, but damp ground gives the butterflies minerals that nectar does not have.

Compared to the eating habits, however, the lifecycle of silver-studded blues and northern blues is much stranger. Their larva eats twigs and legumes, attracting ants with a liquid containing sugar and amino acids excreted from glands specifically evolved for this purpose. These ants then protect the larva from any predators, in exchange for these beads of liquid, and finally become so attached to the larva that they take it into their anthill. Once inside, the butterfly takes advantage of the situation and transforms from a vegetarian into a carnivore: it feeds on the ants’ offspring. Miraculously, the ants will let their destructive guest go after it has grown into an adult butterfly.